The continued interest in Miami mansions and penthouses has as much to do with the rest of the wealthy foreigners' portfolio and liquidity needs as it does the real housing market.

That makes the market especially sensitive to financial shocks from Europe or a slowdown in growth in emerging markets. If European markets tumble and China has a hard landing, the shocks will be felt quickly at the top of Miami's glass-walled Continuum Towers or Setai .

“This is flight to safety for these buyers, an investment decision” said Jonathan Miller of Miller Samuel Real Estate Appraisers and Consultants. “The question is how long and how far will it go? Does this create a bubble at the very high end in these specific markets?”

Mr. Miller said he suspects the strength to continue in Miami for the foreseeable future. But he adds that in the past, high-end markets can turn on a dime.

“It’s like an on-off switch,” he said. “It can shut off quickly.”

-By CNBC's Robert FrankFollow Robert Frank on Twitter: @robtfrank

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